The objective of the work is first the study of the mechanism of action of Vitamine D3 metabolites, on bone. This would be accomplished by testing the effect of Vitamine D3 metabolites on Cyclic-AMP, and Calcium uptake of isolated bone cells in culture. The second part of the proposed study is the Bioassay of 1,25-(OH)2-D3 in plasma, using 45-Ca-prelabeled rat fetal bones, in vitro. A third part of the work would be the study of the Interaction between different Vitamin D3 metabolites and derivatives on the isolated rat fetal bone.